Field of the Invention
The present invention relates to a method for producing a tubular fiber arrangement as an intermediate product for a fiber-reinforced composite part. The tubular fiber arrangement has a cross-sectional shape that changes along its length.
Fiber-reinforced composite parts have been gaining increasing popularity because, as compared to conventional components, they have a comparatively light inherent weight and they can be produced and used in a particularly flexible manner.
However, conventional fiber-reinforced composite parts are associated with a problem in that in the formation of hollow profiles, in which the fibers are arranged on a core, the density of the fiber arrangement fluctuates considerably in accordance with the profile of the core or with the target shape of the fiber composite part, and consequently weak points in the mechanical loadability of what will later be the composite part can often arise.
For example, in the case of material input that exceeds the necessary measure during fiber reinforcement the material costs increase, while this also results in geometric problems, namely when as a result of excessive material input the geometry of the base body, on which base body fiber reinforcement is based, is distorted towards the exterior, in other words is not reproduced in the correct proportions.
German published patent application DE 196 25 798 A1 describes a way to avoid producing a textile tube for a motor vehicle driver, which textile tube comprises reinforcement fibers, by tailoring it with considerable expenditure directly onto a core. There, a textile hose is produced that comprises uneven distribution of the fiber material. The distribution has been preventatively matched to the core shape and which when the hose is pulled onto the core is intended to become uniform. In other words an initially denser fiber arrangement later impinges on bulges of the core, and thus is intended, after being pulled on, to adapt to the density of the fiber arrangement in the remaining region. This is associated with a disadvantage in that subsequently pulling the tube onto the core is precisely unlikely to result in this desired effect because, for example, when passing over larger core diameters, regions are widened that are subsequently intended for a smaller core diameter, and in this process fibers or fiber strands are displaced, which ultimately results in uneven strength and stability and thus in weak points in the composite part produced.
U.S. Pat. No. 4,228,207 and its counterpart German utility model DE 78 26 718 U1 describe the production of a tubular braided fabric with a varying diameter as a flexible formation is known that serves as an intermediate product for rigid but lightweight parts, for example resin-impregnated composite parts such as sports articles or for industrial applications. In order to achieve a desired constant thickness of the braided fabric, in this arrangement the braiding angle, in other words the angle of thread placement relative to the longitudinal axis of the braided fabric, is varied, i.e. for smaller cross sections the braiding angle is reduced by means of an increased transport speed during braiding, while for larger cross sections the braiding angle is increased by means of a decreased transport speed. In this manner the direction of the maximum capacity to absorb tensile forces by the fibers, for example glass threads or carbon threads, which are, for example, used for such components, is changed. Now, assuming that the direction of thread placement is to correspond as far as possible to the direction of load of what will become the composite part, in application of the teaching of this document a more or less significant deviation from the thread placement direction and from the direction of force introduction occurs, which significantly reduces the substance exploitation of the expensive fiber materials and requires over-dimensioning of the fiber arrangement in order to achieve the necessary stability values.
In order to adapt a braiding process to the varying cross-sectional shape of the braided core, we have already proposed (cf. the commonly assigned German patent application DE 10 201 0 047 193 A1) to flexibly adapt to the present core diameter the diameter of the so-called braiding ring that bundles the braiding threads in front of the braiding point. While this does results in a braiding geometry that is more independent of the diameter, and thus results in an equalization of the braiding conditions, it does not, however, solve the problem of the braiding thickness that varies depending on the diameter.